1. Field of the Invention
The present invention relates to a method for functional testing of a mechanical vibration sensor, and to a measuring system for detecting obstacles on a motor vehicle having functional testing.
2. Description of Related Art
Sensor systems for measuring the distance of obstacles, particularly with the aid of ultrasonic sensors, are known from published German patent document DE 10 2005 057 973 and also from U.S. Pat. No. 6,040,765, for instance. In such systems the sensor generates a sound pulse which travels away from the sensor at the propagation rate of the particular medium (usually air), is reflected by an obstacle, and the reflection is then detected by the sensor, whereupon the distance to the obstacle is able to be calculated from the resulting propagation time and the known propagation rate. This distance measurement according to the so-called pulse-echo method requires a periodic excitation of a vibrating component of a vibration sensor. In most cases the excitation of a diaphragm is used on which a piezo element is mounted, the excitation signal defining the frequency of the emitted signal pulse. The reflected pulse is then detected using the same sensor or a different sensor, so that the propagation time is able to be determined from the time interval between the emission of the pulse and the detection of the reflected pulse.
Such measuring systems basically have the inherent problem that no reflected signal is returned to the sensor unit if no obstacles are located in the signal path. A self-test routine for such system is therefore of great importance for ensuring the function of the distance determination or the obstacle detection.
Self-test routines for ultrasonic distance sensors in which reference obstacles such as the ground surface are utilized for generating self-test signals are known from the related art, e.g., published German patent document DE 10 2005 057 973 A1. In those cases it is problematic that the corresponding reference obstacles may change continuously, which means that no clearly reproducible reference signals are available for evaluation.
Another possibility for generating reference test signals is the evaluation of a post-oscillation process of the vibrating component of the vibration sensor, as described in U.S. Pat. No. 6,040,765.
A vibrating sensor with its vibrating component according to the present invention constitutes a harmonic oscillator, which is excited into a forced damped mechanical vibration at the frequency of the excitation signal and preferably at a defined amplitude, using a periodic excitation signal. Depending on its mechanical specifications, a harmonic oscillator has a resonant frequency at which, when being used as excitation frequency, the maximum amplitude is reached in relation to the excitation amplitude. The resonant frequency of a mechanical harmonic oscillator scales reciprocally to the radical of the mass of the vibrating component. To achieve high signal strength, it is therefore advantageous to excite a harmonic oscillator, in this case, the vibrating component of the vibration sensor, at a frequency in the region of the resonant frequency of the harmonic oscillator.
If the periodic excitation signal of a harmonic oscillator is switched off, then the vibrational energy stored in the oscillator is compensated by the damping, such that the oscillator vibration decreases across a decay process. This decay process, the end of which results in the vibration amplitude of zero, will be described in greater detail in the following text.
In the decay process, the harmonic oscillator is vibrating freely, i.e., without being excited into a forced vibrations by an excitation signal, with the result that during the decay process the harmonic oscillator approaches the mechanically specified resonant frequency of the free harmonic oscillator in its oscillation frequency. This decay process is referred to as post-vibration and is directly detectable by the vibration sensor, so that control over the function of the vibration sensor, particularly over the function of the emitted vibration pulse, is able to be obtained. This is attributable to the fact that such vibration sensors are predominantly realized using piezo ultrasonic sensors, so that a vibration component simultaneously may be used as transmitter and as receiver for a corresponding vibration pulse. As soon as the excitation signal is switched off, the sensor is therefore able to be switched into the detection mode so that the post-vibration signal is able to be tapped directly at the sensor as measuring signal.
Such vibration sensors do indeed provide direct feedback regarding their function or the accomplished emission of a sound pulse, but they do not provide any information about the quality of the harmonic oscillator or about a possible change in the operating state, beyond the basic emission of a pulse.
Therefore, it is an object of the present invention to provide a self-test routing and a measuring system for obstacle detection having a self-test routine, which provides detailed information about the state of the vibration component or the vibration sensor within the framework of a self-test routine.